1. Field of the Invention
The present invention relates to a rotary electric device and, more particularly, to a method of balancing an embedded permanent magnet rotor in a permanent magnet type rotary electric device.
2. Description of Related Art
Machines powered by electric motors often use permanent magnet rotor assemblies. With reference to FIG. 1, a typical rotor assembly 1 includes a shaft 3 with an even number of substantially identical recessed and overhung slots 5. Slots 5 define substantially identical generally T-shaped ribs 7 with dovetail surfaces adjacent slots 5. A plurality of generally C-shaped lamination stacks 9 surrounds T-shaped ribs 7. Stacks 9 are made up of individual sheets positioned perpendicular to the axis of rotation of shaft 3. The sheets have edges that abut the overhung or dovetail surface of ribs 7. A plurality of non-magnetic cradles 11 for holding permanent magnets 13 is positioned in each slot 5. Each cradle 11 carries laminated permanent magnets 13. Retainer plates 15 enclose permanent magnets 13, lamination stacks 9 and cradles 11. This embedded permanent magnet rotor is designed to axially assemble magnets 13 in their respective cradles 11 as tightly packed as possible to maximize the power per unit of axial length of rotor 1. However, this configuration does not allow access to the center of rotor 1 for a mid-plane balance correction.
Since current permanent magnet rotor assemblies, such as the one described above, are typically run above their first critical speed, it is essential that a mid-plane balance area be available for balancing the rotor. However, this design does not provide such a mid-plane balance area. Instead, the current process is to balance the shaft at full speed using an end planes and mid-span material removal method of balancing. This method is performed by weighing the parts of the rotor that are to be assembled and then calculating the location of each part to give minimum residual unbalance. Such a method is usually sufficient for an initial balancing of the rotor; however, if a factor changes the balance during use, such as bearing area or shaft end repair, it is very difficult, if not impossible, to rebalance the rotor without disassembling the rotor to make a mid-plane balance correction. Accordingly, the current method to make a mid-plane balance correction after initial assembly is to disassemble permanent magnets 13, lamination stacks 9 and cradles 11 from shaft 3, make the mid-plane balance correction and reassemble permanent magnets 13, lamination stacks 9 and cradles 11 to shaft 3. Such a process is time consuming and costly and creates a safety risk in handling magnet 13 since such magnets 13 are extremely powerful.
Accordingly, a need exists for a method of balancing an embedded permanent magnet rotor that allows for a mid-plane balance correction of the rotor without disassembling the rotor.